Process

ABSTRACT

In a process for treating wastewater from a combined gasification and Fischer-Tropsch (F-T) process, feedstock derived from Municipal Solid Waste or the like is gasified in a reactor (R) and treated in a cleanup unit (C) which generates a first wastewater stream (1st WWT STREAM) containing salts and inorganic pollutants. The first wastewater stream is treated in a treatment unit (T1) to remove inorganic pollutants derived from the syngas. The treatment comprises a) degassing, and subsequently b) neutralising the first wastewater stream before treatment in a Dissolved Air Flotation unit (72c) and filtering in a moving sand bed or similar (72d) to remove solids, and a stripping process to remove ammonia. A second wastewater stream (2nd WWT Stream) containing organic pollutants but being low in salts arises from the F-T process and is treated separately to allow recycling within the F-T process.

The present invention relates to a process for treating wastewater froma gasification process. Gasification processes are used to generatefeedstock for Fischer-Tropsch (F-T) processes for the generation ofhydrocarbon fuels.

The Fischer-Tropsch process is widely used to generate fuels from carbonmonoxide and hydrogen and can be represented by the equation:

(2n+1)H₂ +nCO→C_(n)H_(2n+2) +nH₂O

This reaction is highly exothermic and is catalysed by a Fischer-Tropschcatalyst, typically a cobalt-based catalyst, under conditions ofelevated temperature (typically at least 180° C., eg 200° C. or above)and pressure (eg at least 10 bar). A product mixture is obtained, and ntypically encompasses a range from 10 to 120. It is desirable tominimise methane selectivity, i.e. the proportion of methane (n=1) inthe product mixture, and to maximise the selectivity towards C5 andhigher (n≥5) paraffins, typically to a level of 90% or higher. It isalso desirable to maximise the conversion of carbon monoxide.

The hydrogen and carbon monoxide feedstock is normally synthesis gas.

The synthesis gas may be produced by gasifying a carbonaceous materialat an elevated temperature, for example, about 700° C. or higher. Thecarbonaceous material may comprise any carbon-containing material thatcan be gasified to produce synthesis gas. The carbonaceous material maycomprise biomass (e.g., plant or animal matter, biodegradable waste, andthe like), a food resource (e.g., as corn, soybean, and the like),and/or a non-food resource such as coal (e.g., low grade coal, highgrade coal, clean coal, and the like), oil (e.g., crude oil, heavy oil,tar sand oil, shale oil, and the like), solid waste (e.g., municipalsolid waste, hazardous waste), refuse derived fuel (RDF), tyres,petroleum coke, trash, garbage, biogas, sewage sludge, animal waste,agricultural waste (e.g., corn stover, switch grass, grass clippings),construction demolition materials, plastic materials (e.g., plasticwaste), cotton gin waste, landfill gas, a mixture of two or morethereof, and the like. The carbonaceous material may also be SolidRecovered Fuel (SRF) which is a waste product of relatively highcalorific value typically derived from paper, card, wood, textiles andplastics.

The fresh synthesis gas may be treated to adjust the molar ratio of H₂to CO by steam reforming (eg, a steam methane reforming (SMR) reactionwhere methane is reacted with steam in the presence of a steam methanereforming (SMR) catalyst); partial oxidation; autothermal reforming;carbon dioxide reforming; or a combination of two or more thereof. Inthe present application, such treatment of the synthesis gas isconsidered to be broadly part of the F-T process and any wastewaterstreams resulting from such treatment are considered to be wastewaterstreams from the F-T process rather than from the gasification processas such.

The molar ratio of H₂ to CO in the fresh synthesis gas is desirably inthe range from about 1.6:1 to about 2.2:1, or from about 1.8:1 to about2.10:1, or from about 1.95:1 to about 2.05:1.

The fresh synthesis gas may optionally be combined with a recycled tailgas (eg a recycled FT tail gas), which also contains H₂ and CO, to forma reactant mixture. The tail gas may optionally comprise H₂ and CO witha molar ratio of H₂ to CO in the range from about 0.5:1 to about 2:1, orfrom about 0.6:1 to about 1.8:1, or from about 0.7:1 to about 1.2:1.

The combined FT synthesis gas feed (comprising of fresh synthesis gascombined with recycled tailgas) desirably comprises H₂ and CO in a molarratio in the range from about 1.1 to about 2.1:1, or from about 1.7:1 toabout 2.0:1, or from about 1.7:1 to about 1.9:1.

The invention is concerned particularly but not exclusively withtreating wastewater from a gasification process utilising MunicipalSolid Waste (MSW) or Commercial and Industrial waste (C & I) as thegasification feedstock, which tends to generate wastewater with highlevels of pollutants. The disposal of such wastewater and the removal ofsuch pollutants is of pressing concern.

There is a demand for disposal of MSW and C & I waste which does notinvolve landfill.

There is furthermore a demand for fuel derived from renewable resources.For example, the Renewable Transport Fuel Obligation (RTFO) obligates UKsuppliers of road transport fuels (such as refiners and importers) inexcess of 450,000 litres annually to use a certain percentage ofsustainable biofuels.

It is known to recycle wastewater from an F-T process.

Furthermore, it is known, eg from WO 2017/011025A and WO 2017/039741A totreat separate wastewater streams from gasification and F-T processes ina combined gasification and F-T installation utilising MSW as thefeedstock. However, these patent applications disclose no details of thewastewater treatment or of the pollutants removed from the wastewater.

F-T wastewater treatment is disclosed in WO2016193337A1 which discussedpre-treating the waste water by distillation or steam stripping,removing residual wax by gravity and feeding the resulting pretreatedwaste water to a granular sludge based anaerobic bioreactor. Thisdocument pays scant regard to the treatment of salt water streams exceptby ion exchange or reverse osmosis.

In one aspect the present invention provides a process for treatingwastewater from a combined gasification and Fischer-Tropsch (F-T)process in which aqueous effluent from the gasification is treated withalkali to produce a first wastewater stream and the first wastewaterstream is treated to remove inorganic pollutants present in the aqueouseffluent, and a second wastewater stream, containing water produced inthe F-T process and being distinct from the first wastewater stream, istreated separately from the first wastewater stream to remove organiccompounds.

The treated first wastewater stream may be discharged to theenvironment. The treated second wastewater stream may be reused withinplant utilised in the gasification and/or F-T process.

Thus, the invention also provides a process for treating wastewater froma combined gasification and Fischer-Tropsch (F-T) process in whichaqueous effluent from the gasification is treated with alkali to producea first wastewater stream and the first wastewater stream is treated toremove inorganic pollutants present in the aqueous effluent, and asecond wastewater stream, containing water produced in the F-T processand being distinct from the first wastewater stream, is treatedseparately from the first wastewater stream to remove organic compounds,wherein the treated first wastewater stream is discharged to theenvironment and the treated second wastewater stream is reused withinplant utilised in the gasification and/or F-T process.

This has the advantage that the treatment of the wastewater streams isoptimised.

Salty inorganic wastewaters are separately treated from organic laden,non-salty wastewaters. In preferred embodiments, this allows thenon-salty (fresh) water to be reused within the facility for coolingwater makeup or other resource.

The first wastewater stream may for example comprise treated aqueouseffluent from any one or more of a gasification zone, a partialoxidation zone, a clean-up zone and/or a hydrogen to carbon monoxideratio shifting zone (e.g. a water gas shift zone).

In a preferred embodiment there is provided a process for themanufacture of one or more useful products (such as long chainhydrocarbons for example) comprising:

-   -   a. gasifying a carbonaceous feedstock, preferably comprising        waste materials and/or biomass, in a gasification zone to        generate a raw synthesis gas;    -   b. optionally partially oxidising the raw synthesis gas in a        partial oxidation zone to generate partially oxidised raw        synthesis gas;    -   c. supplying at least a portion of the, optionally partially        oxidised, raw synthesis gas to a clean-up zone to remove        contaminants and provide a clean synthesis gas;    -   d. optionally shifting the hydrogen to carbon monoxide ratio of        the clean synthesis gas in a hydrogen to carbon monoxide ratio        shifting zone to generate shifted clean synthesis gas;    -   e. supplying the, optionally shifted, clean synthesis gas to an        F-T reaction train to generate at least one first useful        product;    -   f. optionally upgrading the first useful product in a second        further reaction train to generate a second useful product,

wherein aqueous effluent from one or more of stages a. to c. is treatedby degassing and subsequent neutralisation and aqueous effluent fromstages d and e. (and optionally also stage f.) is separately treated.

It has been found that the first wastewater stream can normally beeconomically treated to remove pollutants to satisfy regulatoryrequirements, even if the feedstock is derived from MSW or C & I waste.

Preferably the treated first wastewater stream is discharged to theenvironment.

Preferably the treatment comprises:

-   -   a) degassing, and subsequently    -   b) neutralising    -   c) preferably clarifying, and    -   d) preferably filtering

the first wastewater stream.

In a related aspect the invention provides a process for treatingwastewater from a combined gasification and Fischer-Tropsch (F-T)process in which aqueous effluent from the gasification is treated withalkali to produce a first wastewater stream and the first wastewaterstream is treated to remove inorganic pollutants present in the aqueouseffluent, wherein the treatment comprises:

-   -   a) degassing, and subsequently    -   b) neutralising,    -   c) preferably clarifying, and    -   d) preferably filtering

the first wastewater stream.

The preliminary degassing step reduces the requirement forneutralisation and enhances the economics of the process. Acid gasessuch as CO₂ and SO₂ which would otherwise exert a caustic demand arereleased. This also helps maintain a lower salinity in the final treatedeffluent.

Additionally, the wastewater treatment of the present invention, in bothits aspects, has been found to be remarkably effective in reducing heavymetal and other pollutants, even when using a relatively dirty feedstocksuch as MSW or C & I waste.

Preferably the process comprises the further step:

c) oxidising dissolved or suspended components of the neutralised firstwastewater stream.

This facilitates removal of heavy metals, as well as reducing thechemical oxidation demand (COD) of the wastewater.

Preferably the first wastewater stream is neutralised in a reaction zonewhich is agitated by an oxidising gas (eg air).

This ensures complete mixing and hence neutralisation and also enablesneutralisation and oxidation in one and the same reaction vessel.

In a preferred embodiment the reaction zone is agitated by bubbleaeration in the presence of a catalyst, preferably a cobalt catalyst ora ferrous catalyst, for the oxidation of one or more of: sulphites,nitrites and arsenic compounds.

Preferably the first wastewater stream is treated with activated carbon(preferably powdered activated carbon) to absorb organic compoundsand/or heavy metals.

This enables a significant reduction of pollutants in an economicalfashion.

Preferably the treated first wastewater stream is subjected to adissolved air flotation process to separate spent activated carbon andother suspended solids (if present).

This process complements the treatment with activated carbon. Thesuspended solids will typically include heavy metal oxides.

Preferably the first wastewater stream is filtered with a sand filter,multimedia filter or membrane filter, to remove any remaining spentactivated carbon and suspended solids (if present).

This feature enables virtually complete clarification of the wastewaterin an economical fashion.

Preferably the first wastewater stream is treated with a coagulatingagent, preferably an aluminium or iron-based coagulant and/or aflocculation-promoting polymer, to assist in the removal of suspendedsolids.

This feature is particularly advantageous in combination with dissolvedair flotation because it agglomerates the small particles in theeffluent and assists their removal by the dissolved air flotation. Thecoagulant also assists in the capture of heavy metals.

Preferably the first wastewater stream is subject to an air or steamstripping process, preferably under alkaline conditions, to removeammonia. The stripped ammonia is captured and reused within thefacility.

Preferably the first wastewater stream is treated with a sulphidecompound. The sulphide may be an inorganic sulphide such as sodiumsulphide for example, or an organic sulphide compound, preferably aheteroaromatic sulphide, most preferably an S-triazine sulphide salt, toprecipitate heavy metals.

These last two features are particularly advantageous in combinationwhen the first wastewater stream is made alkaline, because this reducesstill further the solubility of precipitated heavy metal complexes.

The invention also provides a plant configured to operate the processdisclosed herein. The plant may be a combined gasification andFischer-Tropsch (F-T) plant.

Other preferred features are defined in the dependent claims.

All the preferred features can be combined in any combination.

Preferably the preferred process steps and combinations thereof areperformed in the order stated above.

A preferred embodiment of the invention is described below by way ofexample only with reference to FIGS. 1 to 4 of the accompanyingdrawings, wherein:

FIG. 1 is a schematic diagram of a Feedstock Conditioning Facility usedto process MSW or C & I waste to a feedstock for a combined gasificationand F-T process;

FIG. 2 is a schematic diagram of a combined gasification and F-T processutilising the feedstock generated by the FCF of FIG. 1;

FIG. 3 is a schematic diagram of the unit T1 (apparatus 72 a-72 e) usedfor treatment of the 1st WWT (Wastewater) stream in FIG. 1, and

FIG. 4 is a schematic diagram showing the degassing tank and reactiontank arrangement of unit T1 in more detail.

FEEDSTOCK CONDITIONING

Referring to FIG. 1, the FCF shown receives bagged C&I and MSW Wastefrom a bunker (not shown) from which the bags of waste are transferredto a bag splitter 1.

The waste from bag splitter 1 is fed to a vibration conveyor c1 whichpasses beneath a belt magnet 2 and an eddy current rotor 3 which removeferrous and non-ferrous metals respectively.

Oversized items are also removed at this stage.

The processed waste then passes to a density separator 4 which removeshigh density materials such as glass and rubble which are notcombustible.

The processed waste is then transferred by a conveyor c2 to fineshredder 5 which reduces the particle size to 25 mm or less.

The size-reduced waste is then transferred by a conveyor c3 to a beltdryer 4 where excess moisture is removed. The dried waste (typicalmoisture content 10 wt %) is then transferred by a conveyor c4 to abunker 7.

Bunker 7 also receives Solid Recovered Fuel (SRF) which is a wasteproduct of somewhat higher calorific value than MSW and C&I waste and istypically derived from paper, card, wood, textiles and plastics.

The combined material from bunker 7 is then transferred by a crane toconveyor assembly c7, which feeds the processed feedstock a baler 8.

Gasification

Referring now to FIG. 2, baled feedstock from baler 8 is fed to a feeder12, which pressurises the feedstock to reactor pressure and feeds it toa gasifier 21 of a reactor assembly R.

Reactor assembly R further comprises a partial oxidation (POx) reactor22 and a radiant cooler 23.

The gasifier 21 comprises a steam reforming reactor incorporating a deepfluidised bed, the bed operating temperature being typically 600-800° C.The fluidised bed is fluidised with superheated steam and causes thecarbonaceous material of the feedstock to pyrolyse and react with thesteam to form hydrogen, carbon monoxide and carbon dioxide.

The syngas product of gasifier 21 is fed to partial oxidation reactor22, which also receives F-T tailgas from an F-T reactor 51 and alsooxygen. Reactor 22 is operated at a temperature above the ash meltingpoint at a sufficient residence time to convert tars and oils andmethane in the syngas to carbon oxides, hydrogen and water.

The syngas output of partial oxidation reactor 22 is fed to a cooler 23which comprises radiant and convective cooler units. Reactor 22 alsogenerates molten ash which is solidified in cooler 23.

The HRSG (heat recovery steam generator) has a blowdown stream of waterwhich contains slag particles from the gasifier and PDX. Theconcentration of suspended solids is relatively high in this stream andit is therefore sent directly to the sludge dewatering centrifuge 72 e(centrifuge rather than cyclone) for removal of the bulk of the solidsbefore the liquid phase is co-treated with the rest of the salty water.

The cooled syngas from cooler 23 is fed to a Venturi scrubber 31 a of agas cleanup unit C, which further comprises an acid gas removal unit 31b, a compressor 41 and an acid gas removal unit 42.

Particulate matter is removed in Venturi scrubber 31 a, and theresulting scrubbed syngas is passed to a halide removal unit 31 b.Halide removal unit 31 b comprises a packed column over which sodiumhydroxide solution is passed to absorb hydrogen chloride, bromide andfluoride. The resulting 1st wastewater (WWT) stream, containing halidesalts, is passed to a degassing tank 72 a of a first water treatmentassembly T1.

The syngas output of halide removal unit 31 b is compressed in acompressor 41 and then cooled, condensing liquid (wastewater) which isthen removed from the syngas and fed to a degassing tank and then on toDissolved Air Flotation (DAF) unit 73 a, discussed below.

The compressed syngas from compressor 41 is fed to acid gas removal unit42, which operates at low temperature and high pressure and usesmethanol as a solvent for removal of hydrogen sulphide, carbonylsulphide, carbon dioxide and trace impurities such as hydrogen cyanide,ammonia, formic acid and metal carbonyls which might otherwise bedetrimental to the downstream process units, in particular by poisoningthe F-T catalyst. Unit 42 preferably utilises the RECTISOL™ process. Thedissolved impurities are removed from the methanol solvent by stepwiseflashing and are passed to an incinerator 45. The acid gas removal unit42 also includes a mercury guard bed for absorption of mercury.

Liquid from the RECTISOL™ process in acid gas removal unit 42 and fromthe shift process in unit 43 is fed via a degassing tank (not shown) toDAF unit 73 a. Acid gas from unit 42 is fed to incinerator 45.

Absorbed carbon dioxide is regenerated and fed to a CO₂ compressor 47,which discharges purified carbon dioxide to the atmosphere and alsogenerates contaminated water which is fed via a degassing tank (notshown) to DAF 73 a.

The syngas output of acid gas removal unit 42 is fed to a shift reactor43 where the hydrogen content of the syngas is increased. Shift reactor42 communicates with a pressure swing adsorption reactor 44 in whichimpurities in the hydrogen such as carbon monoxide, carbon dioxide,methane, nitrogen and argon are removed. Liquid generated in shiftreactor 43 is fed to a degassing tank 72 a and then on to DAF 73 a.

F-T Synthesis

The syngas from reactor 43 is fed via a guard bed 48 to aFischer-Tropsch unit 51. F-T unit 51 comprises three parallel F-Treactors in a train, each made up of an outer shell (pressure vessel)containing 4 microchannel cores. Each core is made up of multiplevertical and cross-flow microchannels.

Water generated in the F-T reaction is fed to a steam stripper 71 of asecond water treatment assembly T2.

F-T products from the F-T unit 51 are fed to a liquid upgrading unit 61,which produces high quality naphtha and Synthetic Paraffinic Kerosene(SPK). The liquid upgrading unit is configured as a recycle hydrocrackerto achieve full conversion of F-T materials while maximizing SPKproduction. This is achieved by hydrocracking, hydroisomerisation, andhydrotreating, using appropriate catalysts.

The output of liquid upgrading unit 61 is fed to a fractionator 62,which generates SPK as the main fuel product. Contaminated water fromfractionation 62 is fed to steam stripper 71.

Treatment of 1st WWT

Referring to FIGS. 2, 3 and 4, the first WWT stream from the Venturiscrubber 31 a is degassed in the degassing tank 72 a. This degassingtank operates under vacuum and, as shown in FIG. 4, is fitted with amulti-tiered cascade system CS to allow gases to escape naturally. Thedegassing tank is fitted with an externally mounted mixer pump MP toprevent suspended solids settling inside the tank. The tank is alsobenched, with the outlet pipework at the lowest point, to prevent solidsaccumulating in the tank.

Off-gas is sent to the incinerator 45, along with other process gases.In the incinerator 45, sulphurous gases are incinerated to sulphurdioxide, and this gas is then scrubbed from the incinerator flue with asodium hydroxide solution before the vent gas is released to atmosphere.

The resulting sodium sulphite/bisulphite solution is also sent toreaction tank 72 b for oxidation to sodium sulphate in the presence of acobalt or ferrous catalyst. Reaction tank 72 b is aerated by means of acoarse bubble aeration system A (FIG. 4) using two blowers. Aerationallows for the oxidation and precipitation of species such assulphites/bisulphites, nitrite and arsenic. Neutralisation of the feedis accomplished by dosing of sodium hydroxide. The aeration also mixesthe tank effectively.

The spent caustic solution contains sodium sulphite and sodiumbisulphite, and this wastewater is combined with the degassed water fromdegassing tank 72 a and fed into a reaction tank 72 b where thewastewater streams are both neutralized with sodium hydroxide andoxidized by aeration. Sulphite is converted to sulphate with the aid ora cobalt or ferrous catalyst. Powdered Activated Carbon (PAC) is alsodosed (see FIGS. 3 and 4) for removal of residual mercaptans followingdegassing, as well as certain heavy metals, phenols, cresols or otherorganics that could be present in the water. Cobalt (II) chloride orferrous chloride catalyst is dosed to catalyse the oxidation of sulphiteto sulphate. This tank as well as the subsequent DAF unit 72 c is odourcontrolled.

Flows then pass to a DAF (dissolved air flotation) unit 72 c. A heavymetal scavenger (TMT-15 or similar) is dosed, along with coagulant andpolymer to improve the capture of heavy metals and suspended solids inthe DAF unit. An aluminium based coagulant is then added to DAF unit 72c via an alum dosing pump to facilitate coagulation.

Washwater from a downstream filtration process, unit 72 d, is also fedto the DAF unit 72 c for clarification. It is assumed that the solids inthe degassed water are finely divided soot particles, washed from thegasifier overhead product. In order to remove these very fine particles,they must be coagulated into larger flocs for easier removal byclarification and filtration.

A polymer, preferably a polyacrylamide anionic polymer, is added to theDAF unit 72 c by a polymer dosing package (not shown) to facilitateflocculation.

TMT-15 (1, 3, 5-triazine-2, 4, 6-triathione sodium salt) or similar, isdosed for precipitation of heavy metals, subject to limits in thedischarge permits. The floc particles are floated to the surface of theDAF unit 72 c. The solids form a sludge which is continuously scraped toa sludge hopper (not shown) for transfer to the sludge dewateringcentrifuge 72 e which generates sludge cake for disposal.

Clarified water from the DAF unit 72 c is then pumped to a filtrationunit 72 d. This provides continuous filtration. The type of filtrationwill be site specific depending on the discharge water qualityrequirements.

Depending on ammonia loading in the wastewater and the relevantdischarge permits, an ammonia stripping system may be required betweenthe DAF unit 72 c and the filtration unit 72 d. Ammonia can be strippedby dosing sodium hydroxide to raise the pH, then counter-currentstripping in a packed tower with either air or steam as the strippingmedium.

The high total dissolved solids (TDS) levels of the filtrate precludesits recycling as cooling water make up. Filtrate is therefore dischargedvia an effluent balancing tank (not shown). Here it is blended withother salty waste streams such as ion exchange softener regenerationbrine and cooling tower blowdown.

The high total dissolved solids (TDS) levels of the filtrate precludesits recycling as cooling water make up. Filtrate is therefore dischargedvia an effluent balancing tank (not shown). Here it is blended withother salty waste streams such as ion exchange softener regenerationbrine and cooling tower blowdown. In this manner the treated water fromthe filtration unit 72 d is safely discharged to the environment.

Sludge from the DAF unit 72 c is dewatered in sludge dewateringcentrifuge 72 e, along with PDX slag/water from unit 230. Centrate fromcentrifuge 72 e is reprocessed in DAF unit 72 c. Clarified water fromDAF unit 72 c is then further polished in a filtration unit 72 d.Ammonia stripping with air or steam may optionally be included here ifrequired by the pollution load and discharge permit conditions. Thefilters (and stripped) water is then sent to an effluent balancing tank(not shown) where it is blended with other saline streams includingcooling water blowdown and softener regeneration brine, before beingdischarged to a suitable watercourse.

Salty wastewater from the scrubber unit 31 a (1st WWT) is routed to adegassing tank 72 a operating under vacuum. Referring again to FIG. 4,the tank is fitted with a multi-tiered cascade system CS to allow gasesto escape naturally.

The reaction tank 72 a is fitted with an externally mounted mixer pumpMP to prevent suspended solids settling inside the tank. The tank isalso benched, with the outlet pipework at the lowest point, to preventsolids accumulating in the tank.

A vent from the tank is routed to the incinerator 45. The degassed wateris passed forward to reaction tank 72 b for neutralisation, oxidationand adsorption. In the incinerator 45, sulphurous gases are incineratedto sulphur dioxide, and this gas is then scrubbed from the incineratorflue with a sodium hydroxide solution. The resulting sodiumsulphite/bisulphite solution is also sent to reaction tank 72 b foroxidation to sodium sulphate in the presence of a cobalt or ferrouscatalyst.

Reaction tank 72 b is aerated by means of a coarse bubble aerationsystem A using two blowers. Aeration allows for the oxidation andprecipitation of species such as sulphites/bisulphites, nitrite andarsenic. Neutralisation of the feed is accomplished by dosing of sodiumhydroxide. The aeration also mixes the tank effectively.

Powdered Activated Carbon (PAC) is also dosed for removal of residualmercaptans following degassing, as well as certain heavy metals,phenols, cresols or other organics that could be present in the water.Cobalt (II) chloride or ferrous chloride catalyst is dosed to catalysethe oxidation of sulphite to sulphate. This tank as well as thesubsequent DAF unit 72 c is odour controlled.

The range of selected contaminants that can be dealt with by first watertreatment assembly T1 are given in Table 1 below.

TABLE 1 Stream 310-102, Gas 450-106, Clean-up Spent POX wastewaterCaustic Slag/Water Total suspended 2,000-20,000  0-100 100,000-500,000 solids, mg/l Total Organic  1-100 1-10 N/A Carbon, mg/l Chemical oxygen 10-1,000 5,000-20,000 N/A demand, mg/l Halides, mg/l 2,000-20,000 0-200 2,000-20,000 Phosphorus, mg/l 0-20 0-20 0-20 Hydrogen sulphide, 100-1,000 5,000-30,000  100-1,000 sulphur dioxide, sulphite ion andbisulphite ion, mg/l as S Ammonia as N, mg/l 20-200 0-10 1,000-50,000Heavy metals*, mg/l  1-100 0-2  10-100 *Includes As, Hg, Ni, Cd, Cu, Pb,Cr, Co, Ga, Mo, V and Zn

Treatment of 2nd WWT

Process water from F-T unit 51 and fractionation unit 62 are sent to thesteam stripper 71, as noted above.

The above combined process water feed stream (2nd WWT STREAM) is firstpreheated and then flows down through a packed/trayed tower strippingsection where it is contacted by rising steam. The flow of steam is setin ratio to the feed flow. The steam volatizes most of the organiccontent of the feed, yielding a bottoms stream of water with smallamounts of hydrocarbons. The bottoms stream is arranged to preheat thefeed stream. The bottoms stream is further cooled in an effluent cooler(not shown).

The cooled stripped water is sent for further treatment to a DAF unit 73b via a DAF feed tank 73 a. DAF feed tank 73 a receives wastewaterstreams from compressor 41, gas removal unit 42, shift reactor 43 andCO2 compressor 47. These additional streams are degassed prior toentering the tank, to release entrained gases including carbon dioxide.

The above DAF assembly removes any remaining free oil from the combinedstream, as well as any residual solids.

The feed is first pH corrected with sodium hydroxide, and subsequentlyfed into the DAF coagulation zone. A coagulant, for example aluminiumsulphate, is dosed to coagulate the solids and oil droplets into largerparticles in order to separate them from the water phase.

Air for the DAF process is supplied by a dedicated compressor (notshown). The air is dissolved under pressure into a recycled water flowin a contactor (not shown) and the aerated water is depressurized as itis mixed with influent feed to produce micro-bubbles of air. The bubblesattach to the coagulated particles and float them to the top of the DAFunit 73 b, where they are removed as sludge by a skimmer (not shown),into a built-in sludge hopper (not shown). The sludge is removed offsite by tanker.

Clarified water from the DAF unit 73 b is pumped to a Membrane BioReactor (MBR) 73 c which is fed with nutrients and converts organicpollutants to microbiological sludge, which may be transferred to asewage works or other off-site or on site sludge treatment facilities.

The purified water from MBR 73 c is dosed with anti-corrosion,anti-microbial and anti-deposition chemicals at dosing unit 84 a andthen fed to a cooling tower 84 b where it is cooled prior to the treatedcooling water being fed to units requiring cooling.

Users of cooling water include:

-   -   ash handling (not shown)    -   gasifier 21    -   gas cleanup unit C    -   shift reactor 43    -   incinerator 45    -   F-T unit 51    -   fractionation 62    -   wastewater treatment units T1 and T2.

1. A process for treating wastewater from a combined gasification andFischer-Tropsch (F-T) process in which aqueous effluent from thegasification is treated with alkali to produce a first wastewater streamand the first wastewater stream is treated to remove inorganicpollutants present in the aqueous effluent, and a second wastewaterstream, containing water produced in the F-T process and being distinctfrom the first wastewater stream, is treated separately from the firstwastewater stream to remove organic compounds, wherein the treated firstwastewater stream is discharged to the environment and the treatedsecond wastewater stream is reused within plant utilised in thegasification and/or F-T process.
 2. A process according to claim 1wherein the treatment comprises: a) degassing, and subsequently b)neutralising the first wastewater stream.
 3. A process according toclaim 2 wherein the treatment further comprises c) clarifying the firstwastewater stream.
 4. A process according to claim 3 wherein thetreatment further comprises d) filtering the first wastewater stream. 5.A process according to claim 2, comprising the further step: c)oxidising dissolved or suspended components of the neutralised firstwastewater stream.
 6. A process according to claim 2 wherein the firstwastewater stream is neutralised in a reaction zone which is agitated byan oxidising gas.
 7. A process according to claim 6 wherein the reactionzone is agitated by bubble aeration in the presence of a catalyst, forthe oxidation of one or more of: sulphites, nitrites and arseniccompounds.
 8. A process according to claim 7 wherein the catalyst is acobalt or ferrous catalyst.
 9. A process according to claim 1 claimwherein the first wastewater stream is treated with activated carbon toabsorb organic compounds and/or heavy metals.
 10. A process according toclaim 9 wherein the treated first wastewater stream is subjected to adissolved air flotation process to separate spent activated carbon andother suspended solids.
 11. A process according to claim 9 wherein thefirst wastewater stream is filtered with a moving bed sand filter, or amultimedia filter, or a membrane filter, to remove any remaining spentactivated carbon and suspended solids.
 12. A process according to claim2 wherein the first wastewater stream is treated with a coagulatingagent to remove suspended solids.
 13. A process according to claim 1wherein the first wastewater stream is subject to an air strippingprocess, or steam stripping process to remove ammonia.
 14. A processaccording to claim 1 wherein the first wastewater stream is treated witha sulphide to precipitate heavy metals.
 15. A process according to claim1 wherein a second wastewater stream, containing water produced in theF-T process and being distinct from the first wastewater stream iscooled and subsequently used for cooling plant utilised in thegasification and/or F-T process.
 16. A process according to claim 1wherein gasses extracted from the first and/or the second wastewaterstream are recycled to one or both of an incinerator and a sulphurscrubber.
 17. A process according to claim 1 wherein a second wastewaterstream, containing water produced in the F-T process and being distinctfrom the first wastewater stream, is subjected to: a) steam stripping toremove volatile organic components, and subsequently b) dissolved airflotation to remove less volatile organic components.
 18. A processaccording to claim 17 wherein the second wastewater stream is treatedwith an aluminium-based coagulant, and/or a flocculation-promotingpolymer, to remove suspended solids.
 19. A process according to claim 15wherein the second wastewater stream is passed through a Membrane BioReactor.
 20. A process according to claim 1 wherein Commercial andindustrial waste (C & I) and/or Municipal Solid Waste (MSI) are treatedto form feedstock for the gasification process.
 21. A process accordingto claim 1 for the manufacture of one or more useful productscomprising: a. gasifying a carbonaceous feedstock, comprising wastematerials and/or biomass, in a gasification zone to generate a rawsynthesis gas; b. optionally partially oxidising the raw synthesis gasin a partial oxidation zone to generate partially oxidised raw synthesisgas; c. supplying at least a portion of the, optionally partiallyoxidised, raw synthesis gas to a clean-up zone to remove contaminantsand provide a clean synthesis gas; d. optionally shifting the hydrogento carbon monoxide ratio of the clean synthesis gas in a hydrogen tocarbon monoxide ratio shifting zone to generate shifted clean synthesisgas; e. supplying the, optionally shifted, clean synthesis gas to a F-Treaction train to generate at least one first useful product; f.optionally upgrading the first useful product in a second furtherreaction train to generate a second useful product, wherein aqueouseffluent from one or more of stages a. to c. is treated by degassing andsubsequent neutralisation and aqueous effluent from stages d. and e.(and optionally also stage f.) is separately treated.
 22. A combinedgasification and Fischer-Tropsch (F-T) plant configured to operate theprocess of claim 1.